This invention relates to the processing of signal information and related time series of such information and more particularly relates to a method for enhancing the signal to noise ratio of seismic exploration data.
The present invention is useful in connection with data gathered in seismic exploration by the well known common depth point shooting technique, the one hundred percent coverage technique or any other seismic data gathering technique. The invention is also applicable to the processing of seismic data derived from the use of nonexplosive seismic energy sources and for the enhancement and analysis of seismic traces forming seismic section displays.
In the process of modern technology's search for petroleum the technique of seismic prospecting is frequently used. In seismic prospecting an acoustic energy source is positioned either on the surface of the earth or in a relatively shallow shot hole and an energy impulse as generated by an explosive or a vibration pattern of preselected shape and frequency content is generated from the source. This acoustic energy penetrates through the multiple layers of material comprising the subterranean portion of the earth. Since the speed of sound is generally different in each layer beneath the surface of the earth, reflections and diffractions of the acoustic energy occur at each interface of such layers. These acoustic energy reflections and diffractions cause energy waves to return toward the surface where they are detected by a plurality of spaced geophones or seismic detectors which generate electrical signals representative of the acoustic energy arrivals at their locations.
The acoustic energy detected by the seismic detectors is generally amplified and then recorded or stored in either analog or digital form on some record medium as a function of time after the seismic shot or energy generation. These data may then be displayed for interpretation in the form of a trace or plot of the amplitude of the reflected seismic energy as a function of time for each of the geophones or group of seismic detectors. Such displays or data may then be processed to interpret the arriving acoustic energy at each seismic detector in terms of the subsurface layering of the earth's structure. This analysis, if performed properly, can disclose the location of subterranean traps which may contain petroleum deposits. Digital data processing equipment is frequently used in such seismic data processing.
The processing of such data has been the subject of numerous patents in the prior art. Several such patents are directed toward enhancing the signal to noise ratio or quality of the signals from the geophones or seismic detectors. This is evidence that noise in seismic data is a problem. One of the primary difficulties in obtaining good seismic data for interpretation is the relative weakness of the electrical signals generated by the geophones or seismic detectors due to reflections, refractions or diffractions in comparison with the noise signals generated at or near the vicinity of the seismic exploration. This noise may be caused by a variety of sources such as surface waves generated in the release of the seismic energy for the exploration. Multiple reflections caused by hard layers near the surface and arriving at the geophone locations simultaneous with the arrival of weaker or relatively weaker seismic reflections from the deeper subterranean formations are also a problem. The appearance of such noise could, if uncorrected, provide erroneous interpretation results of the true seismic data. Of course, other sources of noise such as electronic noise from power lines or communication systems used in the trucks and other equipment used in the seismic exploration, or random acoustic energy noise from other energy sources such as moving vehicles in the region of the seismic exploration may also contribute to this problem.
Accordingly, it is an object of the present invention to provide a method of processing seismic data to enhance the signal to noise ratio of such data.
A further object of the invention is to provide improved data processing methods for removing unwanted noise components from seismic data traces without destroying the information carrying components of the traces.
A still further object of the invention is to provide seismic data processing methods for detecting the coherent features in a seismic trace and for emphasizing the coherent features with respect to the noise contained in the trace.
Yet, another object of the present invention is to provide improved seismic data processing methods wherein the characteristics of adjacent seismic traces are examined for coherent features and the coherent features utilized in turn to selectively discriminate the true seismic signal amplitude from any noise component amplitude present in the traces.
The above and other objects, features and advantages of the invention are provided by seismic data processing methods which operate on common depth point or other seismic data records gathered by the use of either explosive or impulsive energy sources or by vibrator type energy sources to enhance the signal to noise ratio of the traces produced therefrom and to remove the noise portion of the signal from the true coherent feature portion of the signal present on the traces. In the method of the present invention a relatively small portion of a record of seismic traces is examined in each processing increment for coherence by providing a search window which spatially covers from 3 to 9 or more seismic traces across the record and which covers a time interval of adjustable length, but also relatively small with respect to the entire record time length, for any coherent feature present in the traces within this search window. The coherence is detected by a unique cross-correlation technique which includes averaging in the space domain. A particular trace within the search window is defined as a reference trace. The coherent feature of the reference trace and adjacent traces in the window is determined and is used to select the portion of the reference trace which is noise. The noise is then removed from the reference trace and records of both the noise and the coherent signal are provided as outputs from the process. The process is then repeated for other search windows until the entire time duration of each reference trace is so processed. A next reference trace is then chosen and new search windows are provided which are located on this trace. The process is thus repeated until the entire record section of seismic data is processed (i.e., all traces are made reference traces) and the noise removed therefrom.